Powder metallurgy is a process to form near net-shape metal parts from powdered feedstock. The process generally includes forming and/or mixing the powdered metal feedstock, compacting the powder to form a ‘green’ part (an unsintered part with enough cohesion to be handled), and sintering the compacted powder to metallurgically bond the powder particles together to form the desired part. In some techniques, the compacting and sintering are performed concurrently and no intermediate green part is formed. In the sintering process, the powder (normally in the form of a green part) is heated to a temperature significantly below the melting point of the powder. For example, titanium alloys have a melting point near about 1,700° C. and yet may be sintered at about 900° C.-1,500° C.
A common technique for powder metallurgy is direct die pressing. In this technique, a powder is poured into a die press cavity and then pressed at high pressure with a punch (also called a die press or a tool) to form the green part. The die press cavity has at least one open end configured to fit the punch. The die press cavity has essentially the same lateral dimensions (dimensions perpendicular to the punch action) as the punch. Thus, direct die pressing is essentially limited to simpler parts, often deemed 2.5-dimensional parts (as compared to true three dimensional parts). The lateral shape is rather arbitrary, but the axial shape (the shape in the direction of the punch action) is limited to varying levels, or thicknesses, with no undercuts or overhangs in the axial direction, with a zero to positive draft angle.
An alternate technique that is capable of forming true three dimensional parts is metal injection molding. In metal injection molding, the metal powder is mixed with a significant amount of organic binder to create a feedstock, which is heated to soften and/or melt the binder and then injected under pressure into an enclosed cavity in a manner essentially the same as plastic injection molding. After molding, the green part of metal powder and organic binder is ejected or released from the mold. The green part then is processed to remove most of the organic binder (the de-binding process). The result of the de-binding process is called a ‘brown’ part. De-binding typically includes chemically dissolving the organic binder and/or heating the green part to melt, vaporize, burn, and/or pyrolyze the organic binder. The final part is formed by sintering the brown part.
The de-binding process may leave organic inclusions in the brown part (where organic binder was trapped and/or incompletely removed). The organic inclusions in the final part may limit the structural integrity and the mechanical properties of the final part.
Additionally, the de-binding process generally results in significant shrinkage of the part. For example, the feedstock may include 30%-40% (by volume) of organic binder. Hence, the green part would include a similar amount of organic binder. The brown part, after de-binding, therefore may have a volume of about 30%-40% less than the green part. The design of the die cavity and the final part needs to account for the significant shrinkage expected to occur during the de-binding process.
One type of metal that is commonly processed with powder metallurgy is titanium alloy. These alloys typically are very strong, light, heat resistant, and corrosion resistant, and find application in a variety of industrial and consumer products. For example, titanium is used extensively in some aircraft for components such as frames, spars, wing boxes, skins, fasteners, engine components (e.g., thrust outlet sheaths, impellers, stators, and bearings), landing gear, doors, air ducting, and floor decking. However, titanium alloys typically are expensive to produce and to process into finished parts. Powder metallurgy is a useful technique for titanium parts but, for example, direct pressing limits the part complexity (and thus its final application) while metal injection molding is a complex process requiring de-binding, any residual binder anomalies result in degraded sintered part integrity.
Therefore, there is a need for improved powder metallurgy techniques that can produce true three dimensional parts and that avoid the limitations of metal injection molding.